/*
 * Copyright (C) 2011 Jacquet Wong
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 * http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

package com.musicg.dsp;

import com.sun.media.sound.FFT;

/**
 * FFT object, transform amplitudes to frequency intensities
 * 
 * @author Jacquet Wong
 *
 */
public class FastFourierTransform {

	/**
	 * Get the frequency intensities
	 * 
	 * @param amplitudes	amplitudes of the signal
	 * @return	intensities of each frequency unit: mag[frequency_unit]=intensity
	 */
	public double[] getMagnitudes(double[] amplitudes) {

		int sampleSize = amplitudes.length;
		
		// call the fft and transform the complex numbers
		FFT fft = new FFT(sampleSize/2,-1);
		fft.transform(amplitudes);
		// end call the fft and transform the complex numbers
		
		double[] complexNumbers=amplitudes;
		// even indexes (0,2,4,6,...) are real parts
		// odd indexes (1,3,5,7,...) are img parts

		// FFT produces a transformed pair of arrays where the first half of the values represent positive frequency components and the second half represents negative frequency components.
		double[] mag = new double[sampleSize/2];
		for (int i = 0; i < sampleSize; i+=2)
			mag[i/2] = Math.sqrt(complexNumbers[i] * complexNumbers[i]+ complexNumbers[i+1] * complexNumbers[i+1]);
		
		return mag;
	}
	
}
